Semiconductor wafers are typically fabricated with multiple copies of a desired integrated circuit design that will later be separated and made into individual chips. A common technique for forming the circuitry on a semiconductor is photolithography. Part of the photolithography process requires that a special camera focus on the wafer to project an image of the circuit on the wafer. The ability of the camera to focus on the surface of the wafer is often adversely affected by inconsistencies or unevenness in the wafer surface. This sensitivity is accentuated with the current drive toward smaller, more highly integrated circuit designs. Semiconductor wafers are also commonly constructed in layers, where a portion of a circuit is created on a first level and conductive vias are made to connect up to the next level of the circuit. After each layer of the circuit is etched on the wafer, an oxide layer is put down allowing the vias to pass through but covering the rest of the previous circuit level. Each layer of the circuit can create or add unevenness to the wafer that is preferably smoothed out before generating the next circuit layer.
One of the methods for achieving planarization of the surface is chemical mechanical polishing (CMP). CMP is a technique in which a chemical slurry is used along with a polishing pad to polish away materials on a semiconductor wafer. The mechanical movement of the pad relative to the wafer, in combination with the chemical reaction of the slurry disposed between the wafer and the pad, provide the abrasive force with chemical erosion to planarize the exposed surface of the wafer (typically, a layer formed on the wafer), when the wafer is pressed onto the pad. Available CMP systems, commonly called wafer polishers, often use a rotating wafer holder that brings the wafer into contact with a rotary polishing pad moving in the plane of the wafer surface to be planarized. The polishing fluid, such as a chemical polishing agent or slurry containing microabrasives, is applied to the polishing pad to polish the wafer. The wafer holder then presses and rotates the wafer against the rotating polishing pad to polish and planarize the wafer.
Another system used for performing CMP to obtain an effective polishing rate involves linear planarization technology. Instead of a rotating pad, a moving belt is used to linearly move the pad across the wafer surface. The wafer is still rotated for averaging out the local variations, but the planarization uniformity is improved over CMP tools using rotating pads, partly due to the elimination of unequal radial velocities. One example of such a linear polisher is described in U.S. Pat. No. 5,692,947. Unlike the hardened table top of a rotating polisher, linear planarizing tools use linearly moving belts that are integrated with polishing pad material or upon which the pad is disposed. The ability for the belt to flex can cause a change in the pad pressure being exerted on the wafer. When the pressure of the wafer-pad engagement can be controlled, it provides a mechanism for adjusting the planarization rate and/or the polishing profile across the surface of the wafer. A support, such as a fluid platen, can be placed under the belt for use in adjusting the pad pressure being exerted on the wafer. An example of a fluid platen is disclosed in U.S. Pat. No. 5,558,568.
When CMP is employed, it is generally advantageous to monitor the effects of the planarizing process to determine if the process is being performed according to desired specifications. One significant challenge in CMP processing is the ability to process each wafer of a particular type in the same way as all other wafers of that type. In other words, it is a goal of CMP to characterize and maintain a polishing environment for each wafer so that there is substantially no variation in planarization characteristics from one wafer to the next.
In CMP there are several methodologies for determining in situ removal rate and, in some cases, in situ uniformity. There are difficulties, however, in measuring pattern wafer metrics, such as dishing or erosion, in situ. These process performance metrics are generally dependent on the consumables used in the CMP process and their characteristics. Accordingly there is a need for an improved method and system for determining CMP pattern wafer performance.